1. Field of the Invention
The present invention relates to a heat radiating structure for cooling an electronic device such as a processor, e.g., an MPU or an image processor by releasing the heat from the electronic device.
2. Background of the Invention
An electronic device such as a processor (MPU) or an image processor has an increasing degree of integration and operating frequency. Accordingly, an electronic device of this kind generates more heat.
As the electronic device is heated to a higher temperature, it may malfunction or break. It is, therefore, necessary to dissipate heat more efficiently from the electronic device. In the prior art, for example, a heat sink is used as a means for cooling the electronic device.
When heat is to be dissipated through a heat sink, it is necessary to transfer heat from the electronic device to the heat sink. For the most efficient heat transfer the electronic device and the heat sink may closely contact each other. However, an electronic device such as an MPU is constructed to have a circuit in a die of silicon and to emphasize electric characteristics. It is, therefore, difficult for a heat sink for heat radiation to make direct contact with the electronic device.
In the prior art there has been developed a heat sink structure, in which a heat spreader is arranged in close contact with the surface of the electronic device to be contacted by the heat sink. One example of this heat sink structure is schematically shown in FIG. 7. A heat spreader 1, as shown, is made of an anodized metal of aluminum or copper. This heat spreader 1 is constructed to have a flat plate portion 2 to be closely contacted by the electronic device, and a protrusion 3 protruding from the flat plate portion 2. The heat spreader 1 is fixed by fitting the protrusion 3 in a pedestal portion 4 such as a lead frame or substrate.
The heat spreader 1 is fixed on the pedestal portion 4 together with a die 5 including an integrated circuit and is contacted by the surface of the die 5 through a grease or jelly 6 having a high thermal conductivity. Moreover, the heat spreader 1 is made of a material having excellent thermal conductivity such as copper so that it can substantially increase the heat transfer area of the die 5 without damaging the die 5 or its circuit. Thus, the characteristics of heat radiation of the electronic device can be improved by fixing a heat sink 7 in close contact with the heat spreader 1.
In order to improve heat dissipation by using the heat spreader 1, it is preferred to make heat resistance between the die 5 and the heat spreader 1 as low as possible. Therefore, the grease or jelly 6 having high thermal conductivity of the prior art which fills the clearance between the die 5 and the heat spreader 1 is preferably replaced by a join which is made directly between the die 5 and the heat spreader 1 by means of soft solder.
However, the electronic part such as the die 5 is made of a material (e.g., silicon) emphasizing electric characteristics, whereas the heat spreader 1 is made of a material (e.g., copper) emphasizing thermal characteristics, so they have very different coefficients of thermal expansion or linear expansion.
When the temperature of the electronic device rises, therefore, thermal stress may occur between the electronic part such as the die 5 and the heat spreader 1 to cause a separation. In other words, their adhesion may be broken by the thermal stress. As a result, heat resistance between the die 5 and the heat spreader 1 may rise high enough to make it impossible to cool sufficiently the electronic part such as the die 5.